1. Field of the Invention
The present invention relates to a passive optical network system and an optical line terminal, and particularly to a passive optical network system and an optical line terminal in which plural optical network units share an optical transmission line.
2. Description of the Related Art
Passive Optical Network (PON) has Optical Line Terminal (OLT) and plural Optical Network Units (ONU). PON converts a signal from a terminal (PC or the like) connected to ONU to an optical signal, passes the optical signal through ONU, a branch (feeder) optical fiber and then an optical splitter while the optical signal is multiplexed optically (time-divisionally) with a main optical fiber to OLT, and then transmits the multiplexed signal to OLT. OLT receiving the optical signal executes various kinds of signal processing. As a result, in PON, communications from a terminal of some ONU to a terminal of another ONU of the PON concerned or a terminal connected to NW are executed.
Data transmitted from OLT to ONU is called as a downstream signal. The downstream signal is delivered from one main optical fiber to all ONUs through all branch optical fibers which are connected to the main optical fiber through optical splitters. Each ONU takes out only data addressed to the ONU itself from delivered data. Accordingly, a downstream band (data transmission position/time) usable every ONU is allocated to OLT in advance so as to prevent the downstream signal to be occupied by specific ONU.
Furthermore, there is executed band control in which OLT allocates an up band (data transmission position/time) of each ONU so that many data can be fairly transmitted through one optical fiber to as many ONUs as possible in accordance with a request from each ONU user.
On the other hand, a signal transmitted from plural ONUs to OLT is called as an upstream signal. For example, an Ethernet (registered trademark) frame signal is transmitted to the upstream signal and the downstream signal. Plural band allocating units which are called as Logical Link ID (LLID) are allocated to individual ONUs. An up transmission permitting timing is indicated every LLID. For example, there is known prior arts: JP-A-11-122172 entitled as “OPTICAL SUBSCRIBER NETWORK SYSTEM” (patent document 1), and IEEE Std. 802.3-2005, chapter 31, chapter 64 and chapter 65 (non-patent document 1) and IEEE Std. 802.3av-2009 Annex 31A (non-patent document 2).
The line speed of PON is increased, and thus it is possible for a user using PON to transmit/receive a larger capacity of data. Therefore, the number of the kinds of communication services to be used at a time in a PON line increases, and thus all the communication services indispensable to social life such as the Internet communication, telephones, television broadcast, are supplied by one PON optical fiber. However, conversely, when a trouble occurs in the PON system, all the communication services for user sides become scarce. Therefore, it is indispensable to reduce the non-communication time caused by the trouble of the PON system as maximum.
As a countermeasure, it is known a method of providing a protection (backup) line between OLT and ONU and switching to the protection (backup) line when PON trouble occurs. A method represented in FIG. 1 of the patent document 1 is known as an example of the construction.
In general, a circuit constructing a protection (backup) line has the same function as a normal circuit. The circuit is supplied with power under a standby state because it is necessary to quickly switch to the protection (backup) line when a trouble occurs.
Furthermore, increase of the speed of PON follows increase of power consumption. Accordingly, with respect to the circuit constructing the protection (backup) line, it is natural that increase of the speed thereof likewise increases power consumption. Therefore, PON having a protection (backup) line consumes power which is nearly twice as much as PON having no protection (backup) line at maximum per line, so that the increment of the power consumption caused by the increase of the speed increases twice at maximum. As the speed of PON is increased, it makes users perceive more clearly that power to be loaded for the protection (backup) line becomes excessive.
Therefore, as the speed of PON is increased, the protection (backup) line is required more and more as described above, and thus it is expected that mounting of protection (backup) line is more and more unavoidable.